The present invention relates generally to a method and an apparatus for the flexible assembly of components and subassemblies into an assembly and, in particular, to a method and an apparatus for preventing building stresses into a final assembly.
The process of constructing a vehicle body includes forming subassemblies from individual body panel components, forming larger subassemblies from groups of smaller subassemblies, panels and other components, and forming a final body assembly from the larger subassemblies. During the construction process, the components and subassemblies must be held in fixed, predetermined positions while attachment operations such as welding and inserting fasteners are performed. Typically, the components and subassemblies are held at a work or assembly station in fixtures utilizing locators and clamps which are movable to abut predetermined points on a component surface during the attachment operation and to retract from the surface to permit the subassembly to be moved to another assembly station. Since components are manufactured within a tolerance range, the locations of the predetermined points will vary from component to component and from subassembly to subassembly. The fixtures are often adjusted in holding positions to accommodate the various locations of the predetermined points.
Fixtures for holding components and subassemblies can be classified as "dedicated" or "programmable." A dedicated fixture, also known as "hard automation", is designed to accommodate a single component or subassembly and to perform a specific operation or set of operations. Typically, a dedicated fixture is capable of only clamping and unclamping movements necessary to perform pre-selected operations on the component or subassembly and cannot accommodate a different component or subassembly representing a design change or a different model. In contrast, a programmable fixture is capable of a range of movements. Thus, the programmable fixture can be programmed to execute a pre-determined set of movements to accomodate a particular component or subassembly, and a different set of movements to accomodate another component or subassembly.
Many programmable positioners have mechanisms of the serial type, i.e., each link of the mechanism is serially mounted on the preceding link forming a chain of links. The links may have linear or rotary joints. Mechanisms with linear joints provide higher rigidity but are still appreciably less rigid when compared with conventional hard tooling fixtures. Programmable fixtures of the serial type are described in the U.S. Pat. No. 5,272,805 (see the FIG. 7 and the FIG. 8) which patent is assigned to the assignee of the present invention.
The present invention provides a programmable positioner with a high degree of rigidity as compared with the rigidity of hard tooling fixtures. It realizes its objective by adopting a parallel linkage mechanism formed by a plurality of linear actuators. The rigidity, or stiffness, of the mechanism is determined by the cooperative supporting structure of the parallel linkage wherein the total stiffness of the mechanism is the sum of the stiffening effect of all the links. Mechanisms of this type have been utilized for flight simulators, well known as the "Stewart Platform"--Stewart, The Institute of Mechanical Engineers, Proceedings 1965-1966, pp. 371-394; and for the construction of machine tools as described, for example, in the U.S. Pat. No. 5,354,158 (also, the U.S. Pat. No. 4,988,244 and the U.S. Pat. No. 5,388,935).
One of the most flexible of fixtures is a 6-axis machine tool. A support includes a pair of spaced platforms joined by six powered and extensible legs, which may or may not be crossed, attached to the platforms by universal joints. In one form, the machine tool has an operator, such as a tool in a spindle, mounted on one of the platforms and an object, such as a workpiece, mounted on the other one of the platforms such that the work space is located between the platforms. The length of the legs is individually manipulated to vary the positions of the platforms and, therefore, the object and operator relative to each other.
The U.S. Pat. No. 4,988,244 shows a machine tool having spaced platforms for mounting a tool and a workpiece which platforms are joined by six extensible legs attached to the platforms in three pairs by clevis and trunnion joints. The pairs of legs are crossed and the process operation is performed between the platforms.
The U.S. Pat. No. 5,388,935 shows a machine tool having spaced platforms for mounting a tool and a workpiece which platforms are joined by six extensible legs attached to the platforms by universal joints. At least one of the pairs of legs is crossed and the process operation is performed between the platforms.
The parallel linkage of the above described type of fixture, wherein the process work is done between the two support platforms, is not suitable for work done above the support, such as in assembly operations. The travel restrictions dictated by such a structure are not supportive of flexible assembly wherein the assembly may be performed on relatively large parts that cannot be contained between the two supports. In another type of device, also including a pair of spaced platforms joined by six extensible legs, the tool is mounted on one of the platforms but faces away from the other platform. The object is mounted on a fixture located relative to the one platform. Thus, the other platform serves as a fixed base in order to move the tool toward and away from the object on which an operation is to be performed.
The U.S. Pat. No. 4,536,690 shows a self-propelled robot platform having a support structure which includes a base and a movable tool support joined by six extensible legs wherein the work space is outside the area between the base and the support. Position control feedback utilizes position and velocity sensors.
The U.S. Pat. No. 5,053,687 shows an articulated device having spaced top and bottom plates joined by six extensible link members with a work space outside the area between the plates. The bottom plate fixedly is supported above a base plate. The link members have top ends attached to the top plate in three pairs and bottom ends attached to six slider members by universal joints. The slider members extend through apertures in the bottom plate to engage endless screws mounted on the base plate. Each slider member includes a shock absorber and is connected to position sensor and each link member includes a force sensor. When force is applied to the top plate, the shock absorbers provide passive compliance and a control utilizes the force sensor information in a force return algorithm to calculate corrections to be applied to the position of the top plate.
Whether dedicated or programmable fixtures are utilized, the components or subassemblies are rigidly clamped on the fixtures which are designed for maximum support rigidity. If the process equipment, such as a spot welding gun, is misaligned, it tends to displace the component or components from the desired assembly location. Accordingly, the components may shift relative to each other causing inaccuracy in the assembly, or, the components are caused to stretch or compress against the rigid fixtures before they are permanently welded. Thus, a problem with existing fixtures is that the assemblies are assembled with internal stresses that could cause failure of the assemblies when in use.